In the Rel-6 specification of 3GPP, an international standardization organization for the 3rd generation mobile communication systems, a multicast type service providing method is specified. In multicast type service providing methods, if multiple mobile stations within the same base station request a certain service (contents), instead of the certain contents being separately transmitted in assigned respective radio channels, the contents are transmitted by the base station in a downlink common channel and are simultaneously received in the common channel by all mobile stations that desire the reception. Such a feature may be generally referred to as “Multicast Broadcast Multimedia Service (MBMS)”. The MBMS will be outlined below.
In the MBMS, as illustrated in FIG. 1, point-to-multipoint traffic channel (MTCH) data are transmitted in a secondary common control physical channel (S-CCPCH) from a base station in the transmission time interval (TTI) of 40 ms or 80 ms. In this example, the MTCH is configured as a logical channel where simultaneous contents are mapped. Also, the TTI represents the unit of radio transmission time of a channel-encoded data block, and the S-CCPCH is configured as a downlink physical channel for radio transmission of the MTCH.
Then, an exemplary inter-frequency measurement leading to degraded quality in the Rel-6 MBMS of the 3GPP will be described below. As illustrated in FIG. 2, if a radio network cannot cover all areas in the same frequency band, but if it can cover all the area using different frequency bands, a base station in one of the areas broadcasts to mobile stations that the residing cell is in a inter-frequency measurement area. In FIG. 2, the notations “f1” and “f2” represent different frequency bands, and some shaded cells correspond to inter-frequency measurement areas. If the reception power level falls below a broadcast threshold, mobile stations in these cells perform inter-frequency measurement.
In the inter-frequency measurement, a mobile station must receive a signal in a frequency band different from the frequency band received in its residing cell. Then, if the mobile station includes two RF paths as shown in FIG. 3A, the mobile station can perform the inter-frequency measurement simultaneously while receiving a signal destined for itself. However, installing such multiple RF paths may increase the cost of the mobile station. In practice, as illustrated in FIG. 3B, a single RF path is thus provided. Alternatively, the mobile station may perform the reception or measurement by tuning a frequency synthesizer to a desired carrier frequency; such a mobile station may be used in practice. In the inter-frequency measurement, the mobile station may tune a common frequency synthesizer to a desired frequency and receive no signal from its residing cell during the measurement.
As mentioned above, when a mobile station having a single RF path performs the inter-frequency measurement, it cannot receive any signal from its residing cell. Thus, a base station must know the inter-frequency measurement timings of the respective mobile stations and prevent downlink user data from being transmitted (DTX: Discontinuous Transmission) while the mobile stations are performing the inter-frequency measurement. In the 3GPP Rel-6 specification, the timing of the inter-frequency measurement by a mobile station is defined in accordance with the following formula;SFN div N=C_RNTI mod M_REP+n*M_REP.
The mobile station performs the inter-frequency measurement under the system frame number (SFN) satisfying the above formula. In the formula, “SFN” represents a frame number, “mod” represents a modulo operation, and “C_RNTI” represents a mobile station identifier assigned for the mobile stations in each cell. The parameters “N”, “M_REP” and “n” are fixed. Thus, the inter-frequency measurement timings of the mobile stations are distributed in each cell in accordance with the above formula. In the formula, it is assumed that data for individual mobile station are transmitted from the base station to the respective mobile stations. In other words, as illustrated in FIG. 4A, when a certain mobile station performs the inter-frequency measurement, data reception by the other mobile stations is enabled. As a result, it is possible to prevent occurrence of any frame where data are not transmitted in the downlink. In non-patent document 1, this type of background technique is described.
As mentioned above, some method for distributing the timings of the inter-frequency measurements by mobile stations by performing some operation on the identifiers of the respective mobile stations may be suitable for transmission of dedicated data. However, such a method may be problematic if a multicast service is provided to multiple mobile stations in the form of multicast such as MBMS. In fact, since the mobile stations start the inter-frequency measurement at the respective timings, there may be no or little time period in which all mobile stations belonging to the same multicast group are not performing the inter-frequency measurement. For this reason, even if the base station transmits MBMS data, some collision may occur between the data transmission and the inter-frequency measurement, and a mobile station entering the inter-frequency measurement may not be able to receive a portion of a radio signal. Since MBMS data are channel-encoded in units of 40 ms or 80 ms and the different frequency measurement is performed in units of 10 ms, the mobile station may not be able to receive all data blocks transmitted during the different frequency measurement. In order to keep the loss rate of the data blocks equivalent to that in cells without the different frequency measurement, however, some compensation may be needed by such as increasing the transmission power of the base station corresponding to the transmission of the MBMS data. This situation is illustrated in FIG. 4B.
Non-patent document 1: 3GPP TS 25.331 V6.4.0 (2004-12), 8.5.11, pp. 243